But why get a module and a function just for this two expression idiom?

I guess the main reason would be that it would be fairly self-evident, even to a Perl newbie, what File::Slurp does and how to use it. I think the first time someone comes across the code in the OP (and perhaps the second and third times as well), they will find it a bit mystifying. Remembering how to use it in their own code could also be a bit difficult.

That's not to say that there's an overwhelming reason to use File::Slurp, just that it could certainly be justified for reasons of code clarity.

Disclaimer: I've never used File::Slurp. I'm assuming that it doesn't suck. :-)